Acoustic absorber pad



May 25, 1955 F. E. CQYLEETAL 3,185,868

ACOUSTIC ABsoRBER PAD Filed May 28, 1962 IN V EN TOR.

564,557 fv- 76 l BY ffm/f 27m: 65.5

/g' mum United States Patent O 3,185,868 ACGUSTIC ABSRBER PAD Forrest E.Coyle, Glen Burnie, and Frank P. Hodges,

Baltimore, Md., assignors, by mesne assignments, to the United States ofAmerica as represented by the Secretary of the Navy Filed May 28, 1962,Ser. No. 198,382

. 3 Claims. (Cl. S10-8.2)

This invention relates to transducer devices and particularly to soundabsorbing medium for such devices. When electrical energy is applied toa piezoelectric crystal of a transducer, 'equal amounts of sound energyradiate from each side of the crystal. In a sonar transducer it is oftendesired to have the energy radiate from vone side to 4obtain directivityand therefore radiated unwanted encountered energy must be absorbed. Asituation where this is encountered is a sonar transducer used atsubstantial depth to obtain information relative to the wave motion inorder to predict the trajectory of a missile fired from an underwaterposition.

It is, therefore, an object of this invention to provide a transducerwith improved directivity.

Another object is to provide a sonar transducer with a new and improvedsound absorbing pad to enable it to obtain better directivity.

A further object is to provide a new material for absorbing highfrequency sound waves.

Still a further object is to provide a method of making a high frequencysound absorbing pad for use in transducers.

Other objects will become apparent from the specicat-ion and drawing inwhich the characteristics of thev invention are set out.

In the drawing,

FIG. 1 is a plan view of a transducer employing the invention.

FIG. 2 is a cross-sectional view taken along the line 2--2 of FIG. 1.

FIG. 3 is -a cross-section through the sound absorbing pad.

The invention is based upon the discovery that high frequency soundwaves are absorbed by a batt of loose polyester bers containing a properproportion of copper chips `distributed through it. A batt of this typeformed into a pad and placed to one side of a wave emitting transducerelement will absorb the emissions from that side and, consequently,directivity is obtained in that emission only from the free side.

Referring to FIGS. 1 and 2, 10 indicates a transducer having a hollowcylindrical body 12, externally threaded at 14, and with a bottom or endwall 16. Transducer is adapted to be joined by threads 14 with anunshown stuiiing tube extending from an underwater craft; an O-ring 18in end wall 16 seals the joint between them. Conductors connect t-hetransducer through the stuing tube to the apparatus within the craft.The position of the transducer on the stuling box is in a upward lookingdirection toward the waters surface. The construction of .a receivingtransducer used in conjunction with the transmitter, for the inventionspurposes, is similar and may, if so desired, employ the sound absorbingpad, shown in FIG. 3, to be described hereinafter. Description, however,is limited to a transmitter transducer.

rThe body 12 of the transducer is generally cylindrical and has an axialcavity 20 opening towards its top. Two apertures, 21, penetrating theend wall 16 and longitudinal flutes 24 in the peripheral wall deningcavity 20 provide a passageway for leading electric energy to theoscillating element within the transducer body.

Seated within the transducer body is a co-axial stack of cylindricalelements shaped to tit the bodys cavity rice 20. The lowermost of theseis a coil supporting cartridge 26. Its lower end is externally reducedby an annular groove 28 in order not to cover the end wall apertures,21, and thereby close t-he passageway between them and flutes 24. Theupper end of coil supporting cartridge 26 has a bore 30 and there is aradial groove 32 in the bottom wall 34 defining the bore 30 whichreceives a coil 36. The combined depth of bore 30 and groove 32 is equalto the height of coil 36. A disk-like coil cover 37 seated on coilsupport cartridge 26 keeps coil 36 in its desired position. Radialgroove 38 in the upper end face of coil cartridge 26 furnishes accessfor conductors from utes 24 to coil 36.

A sound absorbing pad 40 tted into a ring 42, having radial apertures43, is seated on coil cover 37. Each of the elements, ring 42, coilcover 37 and coil support cartridge 26 has Yan external longitudinalgroove, designated .as 35, 35 and 35", respectively, for receiving adowel 43 mounted in one of the flutes 24. This secures them againstrotation within the transducer body cavity 20.

The piezoelectric oscillator element 48 is a ceramic disk of bariumtitanate encircled by a spacer ring 50. It .is disposed between lowerand upper electrical contact rings, 44 and 46, and seated on the soundabsorber assembly, ring 42 and pad 40. Contact rings 44 and 46 areprovided with external, radial ta-bs 47 for entering a ute 24 andsecuring them against rotation. A conductor 51, passing along a flute24, connects contact ring 44 to a connector 45 in transducer bodyaperture 21; whereas, a conductor 52 similarly connects, not shown,contact ring 46 through coil 36 to a connector 47 in the other aperture.Connectors 45 and 47 are adapted to seal the apertures against tiuidpassage and to receive alternating energy from a source, not shown, fortransmission of the energy to the oscillator element by the conductorsinthe utes.

The elements thus far described are held against axial movement intransducer body 12 by lan externally threaded ring nut 54 which isthreaded into the bodys cavity 20 and seated on a shim ring 56. Shim 56is secured against rotation by a tab 57 entering a flute 24.

Transducer body 12 contains oil, not shown, and is closed by a capassembly 58. The latter includes a rubber disk or sound transmittingwindow `60 secured at its ends between upper and lower clamping rings,62 and 64, which are seated on the end face 66 of the trans- -ducerbody. Rings 62 and 64 are carried in an encircling nut 68 having aninward flange 70 that engages an end face of the upper ring 62 yso thatthey move axially with it. Nut 68 is threaded to the external threads 14on the transducer body and further secured with a set screw 70. AnO-ring 72 under the lower ring 64 in the end face of the transducer bodyis furnished as a seal against fluid leakage.

In operation, alternating electric energy, applied at the faces `ofoscillator element 48 through the contact rings, 44 and 46, cause it tovibrate and emit a sound signal in both axial directions of thetransducer body 12. 'Ihe emission from the upper face passes through theoil, the window 60, and the water in which the transducer is immersedtoward the waters surface. An echo results which is rellected tosuitable equipment for resolution. The energy radiated from the lowerface toward the bottom of the transducer body, however, is confronted bythe lsound. absorbing pad 40 and absorbed. Emission is thus in onedirection, upward from the transducer. For the purposes set out,oscillation is one megacycle or ten times normal frequency which is morethan a standard acoustic material can cope with.

The use of various types of lossy rubber and cork to absorb unwantedacoustical energy that is radiated from transducer elements, may besatisfactory for depths and pressure experienced by submarines of WorldWar II vintage. However, at higher pressures, such as those resultingfrom dept-hs within the capabilities of present day submarines, thesematerials lose their lossy properties until this pressure is reduced. Inluid filled transducers, the problem is more severe because the highpressure forces the iluid into the material and then causes a permanentchange.

Absorbing pad 40, FIG. 3, which has proven to perform excellently,comprises a lluffiy batt 76 of polyester fibers of ethylene glycol andterephthalic lacid, known as Dacron in the art and supplied by the E. I.du Pont Company, having interspersed throughout a multitude of curledcopper chips 78. A nylon mesh covering 80 encases it to contain themetal chips and yet allow passage Vof the transducer fluid. The metalchips are used to disperse acoustical energy. The fibers of the Dacronare not bound to each other and proved a pad which is firm and of stabledimension form for suspension of the metal chips are used to disperseacoustical energy.

It will be understood that various changes in the details which havebeen herein described and illustrated, in order to explain theinvention, may be made by those skilled in the art within the principleand scope of the invention as expressed -in the appended claims.

What is claimed is:

l. In an electromechanical transducer, a disk-shaped piezoelectricelement, and a fibrous sound absorbing pad, said sound absorbing padcomprising a batt of fibers of polyester of ethylene glycol andterephthalic acid with curled copper lings distributed therethrough.

2. The device of claim 1 wherein said copper filings are curled andthere is three and one half times their weight per unit weight of saidfibers.

3. The device of claim 2 wherein said pad is enclosed in a nylon meshcasing.

References Cited by the Examiner UNITED STATES PATENTS 2,427,348 9/ 47Bond et al.

2,707,755 5/55 Hardie et al. 3l08.2 2,881,336 4/59 Elion 310-8.22,972,068 2/61 Howry et al.

MILTON O. HIRSHFIELD, Primary Examiner.

ORIS L. RADER, Examiner.

1. IN AN ELECTROMECHANICAL TRANSDUCER, A DISK-SHAPED PIEZOELECTRICELEMENT, AND A FIBROUS SOUND ABSORBING PAD, SAID SOUND ABSOIRBING PADCOMPRISING A BATT OF FIBERS OF POLYESTER OF ETHYLENE GLYCOL ANDTEREPHTHALIC ACID WITH CURLED COPPER FILINGS DISTRUTED THERETHROUGH.